The truth about night vision

As a child did your parents say that if you ate up your carrots you would be able to see in the dark? They weren't exactly lying.

Eating carrots provides us with vitamin A and without this our night vision, the ability to see in dim or low lighting, would deteriorate

There are many other underlying health reasons for poor night vision such as the development of cataracts, diabetes (high blood sugar is toxic to nerves and blood vessels in the eye) and astigmatism.

The bad news is that night vision also deteriorates with age. And while there are many conditions that can temporarily worsen our ability to see well at night including too much sunlight, screen time and smoking, there are plenty of things we can do during the day to improve how well we see at night.

Why our eyes prefer bright to night

It's easy to understand why the eye finds it hard to adapt to seeing in the dark. All light must reach the back of the eye – the retina before we can see.

The retina contains light sensing receptor cells called rods and cones. The rods can only see black and white and the cones can see detail but only in bright light.

Rods and cones contain photopigments, a chemical that converts light energy to electrical energy which is then sent to the brain and interpreted as vision.

Our pupils dilate to let in more or less light as needed - smaller in bright light - and larger in low light. Both rods and cones need to be recharged after exposure to light, otherwise a new signal cannot be sent to the brain.

10 days on the beach equals bad night vision

The longer eyes are exposed to sunlight the longer they need to recover. Have you ever come indoors from being in the sunshine and found it hard to see in the 'darkness'?

Incredibly, just a few hours of exposure to bright sunlight reduces the ability to adjust to the dark by 10 minutes, while 10 days of exposure can cause a 50 per cent (temporary) loss of night vision.

Night blindness nightmare

The most extreme inability to see in dark or low light is called night blindness or nyctalopia. Although rare it can be debilitating for the sufferer and there are no known cures. Causes include retinitis pigmentosa (vitamin A deficiency), glaucoma, hereditary disease or reaction to certain drugs.

Librarian Susie Fuller, began noticing symptoms of night blindness in her 20s when headlights of oncoming cars would startle her vision. After having congenital cataracts removed in her 40s, she had hoped the condition might improve. However, Susie continues to arrange her life around the condition so leaves the office before dark and relies on others to drive her at night. She says: 'It's devastating. I can't drive at night as oncoming headlights simply blind me and I would crash.'

Dr Scott Robbie from The London Eye Hospital explains that although there is no cure for night blindness there is progress. 'Those with a genetic cause for their night vision have varying clinical courses – some remain stable for their whole lives, others may experience a deterioration in the quality of their vision that is not salvageable. Therapies using stem cells and gene therapy are being explored to restore vision in these subjects.'

Young can have poor vision too

According to Dr. Robbie genetic conditions affecting the photoreceptors in the retina can begin with poor night vision and start at a relatively young age. 'It is characterised by poor adaption to dim lighting and difficulties with orientation and mobility in dimly lit environments,' he says. 'People may also have difficulty making out stars in the sky or finding their seat in a cinema.'

Why astigmatism is a culprit

Those with astigmatism may suffer from bad night vision, especially affected by glare from bright lights resulting in halos and poor vision. In a normal shaped eye, corneas should be spherical so that when light passes through to the retina the rays all converge at a single point at the back of the eye.

With astigmatism the cornea is shaped more like a rugby ball and the light rays end up meeting at different points causing blurred vision.

So in lower light the pupil dilates to allow in more light but as the pupil gets larger more peripheral light rays enter causing further visual disruptions.

Learn from pirates – and pilots!

Wear sunglasses

Too much sunlight is detrimental to night vision. Sunlight is a billion times brighter than the lowest light you can still see in, so wearing sunglasses during the day is essential for maintaining good night vision.

Eyes need at least 25 minutes to recover from exposure to bright sunlight to function in darker light. It is widely believed that pirates wore an eye patch so they always had one good eye ready for seeing at night.

Wear red-tinted sunglasses

Wearing red-tinted sunglasses for 30 minutes before entering low light will help eyes adjust more efficiently. Rod cells do not pick up the colour red, therefore the eye blocks out everything except red and the rods adjust faster. That's why some pilots wear red-tinted sunglasses before flying at night.

No smartphones at night

Any bright light is going to hinder your ability to adjust to seeing well at night. Just as with sunlight restrict your exposure by dimming the screen brightness which will lessen the damage to your eyes.

Recent studies have also shown that using a mobile phone at night is bad for overall vision as the blue light from the screen is damaging to the retina.

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